This invention relates generally to sensors for detecting electrically conductive particulate matter, such as soot, and more particularly to a method and system for diagnosing potential failure modes in such sensors.
Incomplete combustion of certain heavy hydrocarbon compounds, such as heavy oils, diesel fuel, and the like may lead to particulate formation (e.g., soot). In the operation of internal combustion engines, excessive particulate formation can lead to “smoking” of the engine, which causes air pollution even though the carbon monoxide, hydrocarbons, and other pollutant components of the gaseous state exhaust emissions may be relatively low. Emission regulations require many engines to limit the levels of particulate emissions, and various control technologies such as diesel particulate filters (DPF) have been employed for this purpose.
In order to monitor the emission of particulate matter in the exhaust streams of certain types of internal combustion engines, e.g., to assess the effectiveness of DPF's, it is known to provide a particulate sensor system for detecting the level of particulate concentration emitted from an exhaust gas. Various particulate sensors have been proposed, including those shown in U.S. Pat. No. 4,656,832 issued to Yukihisa et al., U.S. Pat. No. 6,634,210 issued to Bosch et al., U.S. Pat. No. 7,954,230 issued to Nelson et al., U.S. Pat. Publ. No. 2008/0283398 A1, U.S. Pat. Publ. No. 2008/0282769 A1, U.S. Patent Application Publication No. 2012/0119759 A1, and U.S. Patent Application Publication No. 2013/0002271 A1, the disclosures of each of which are hereby incorporated by reference in their entirety.
Particulate sensors such as those described above generally have a pair of spaced apart sensing electrodes disposed on a substrate. The sensing electrodes are coupled to a measurement circuit by way of electrically conductive leads. The operating principle of the particulate sensor is based on the conductivity of the particulates (e.g., soot) deposited between the sensing electrodes. The electrical resistance between the sensing electrodes is relatively high when the sensor is clean but such resistance decreases as soot particulates accumulate. These sensors also have a heater that can be selectively activated to burn off the soot particulates to “reset” the sensor to a known, base “clean” state.
Regulatory agencies may require that a particulate sensor system has self-diagnostic capability to identify a failure of the particulate sensor to perform its primary function of measuring soot. However, for diagnostic purposes, it can be difficult to distinguish between various states that may occur during various engine operating conditions, such as between: (i) a faulty state such as when the sensor is “poisoned” by a non-conductive or semi-conductive contaminant deposited on the electrodes preventing soot from contacting the electrodes, which presents as a very high resistance between the sensing electrodes, and (ii) a normal state, such as when a sensor has just been cleaned and the DPF is working properly (i.e. preventing soot from passing through to the sensor), which also presents as a very high resistance.
Accordingly, there is a need for particulate sensor diagnostics that can accurately verify particulate sensor operation with a properly operating DPF.